Planet definitions

When is a planet a planet and can Pluto now come back please. http://www.blastr.com/2017-2-21/redefining-planets-answer-search-questionBut the Moon a planet? This definition is going to be just as confusing as the last one. In my mind, a planet is a round world, but not massive enough for nuclear fusion. A moon is something orbiting such a world. But now where do you draw the line where size is concerned, and like this article points out, should there be a line? Are all Saturn's ring particles (down to dust size) moons?

We need a definition, because we have to determine exactly how many planets the solar system has so that we can make kids memorize that number! We can't have them think about concepts and in the process perhaps learn to think for themselves!

I'm beginning to realize I was more serious than I thought at the time, when I joked about school kids needing to memorize the number of planets, and therefore we need a precise definition so we can determine a number.

It's not just school kids either. Most people simply do not have time to go into the subtleties of subjects, and want short and simplified answers that more or less reflect the broad consensus. In at least some subjects, I certainly do. And now it turned out that the solar system has so much variety that something that used to look straightforward has become a bit problematic.

This is how it goes with definitions: sooner or later, nature almost invariably laughs at our definitions and categories.

Pluto's status as a "full planet" seems problematic to me, considering that there may be hundreds of similar bodies out there, and it somehow seems wrong to call them all planets in the same sense as earth or Jupiter. But as has been pointed out, our current definition of dwarf planet has its own problems, and who knows what we might run into as we learn more about other solar systems.

But it seems to me that for the purposes of pleasing school curricula and the general public, one can proceed in somewhat arbitrary manner: the solar system has eight major planets, and an unknown but probably quite large number of minor planets; the latter are mostly, but not invariably, icy. And there you go. I'm not sure we need to agonize about it much more than that.

For scientific purposes, I'm not sure we need a very precise definition to begin with, because on the whole, astronomers know what it is that they are talking about. I seem to often hear them using the term "world", as in "Pluto is a small icy world", or "the asteroids are mostly small, rocky or metallic worlds" or "Jupiter is a Jovian world." Is that common, or was it just Carl Sagan who had such a romantic outlook?

the solar system has eight major planets, and an unknown but probably quite large number of minor planets; the latter are mostly, but not invariably, icy. And there you go. I'm not sure we need to agonize about it much more than that.

Brilliant, Sir, although that succinct summary caused a few "but...umm" moments in quick succession over here.

I seem to often hear them using the term "world", as in "Pluto is a small icy world", or "the asteroids are mostly small, rocky or metallic worlds" or "Jupiter is a Jovian world." Is that common, or was it just Carl Sagan who had such a romantic outlook?

I never noticed this, so I scratched around a little and experienced another one of those moments. Before I discard my work for the day and get sucked into the internet black hole I am going to go with: "we can cope with having three capitals, so we can deal with this world word!" I suppose I can use my 'hackles test'. It will raise my hackles to hear of someone referring to Cape Town as the capital of South Africa. My hackles will stay down if they specified legislative capital. So, the context would matter.